One of the small-but-fascinating surprises from yesterday's Facebook earnings call was CEO Mark Zuckerberg's impromptu discussion of teens on Facebook, and whether they're abandoning the social network.

Yes, Zuck cares whether the kids think he's cool.

The issue here is that one of Facebook's big downside risks is the day when Facebook suddenly becomes so uncool that no one wants to use it. That was the MySpace story, basically. Once teenagers made a collective decision that there were too many ads/parents/old people on MySpace, they all left, quickly followed by everyone else. In May, the Pew Research Center released a study that showed teens were increasingly negative on Facebook.

But Zuckerberg said yesterday that teens are not leaving Facebook, and in fact the company has close to 100% of all teens on the service:

One specific demographic I want to address is U.S. teens. There's been a lot of speculation and reporting that fewer teens are using Facebook, but based on our data that just isn't true. It's difficult to measure this perfectly since some young people lie about their age, but based on the best data we have we believe that we are close to fully penetrated in the U.S. teen demographic for a while and the number of teens using Facebook on both a daily and monthly basis has been steady over the past year and a half. Teens also remain really highly engaged using Facebook. Now it's also worth mentioning that these stats are for Facebook only. Instagram is growing quickly as well. So if you combine the two services together, we believe our engagement and share of time spent are likely growing quickly throughout the world."

Note that he doesn't address whether teens like Facebook or not, just that they're still using it. Another interpretation of this statement is that Facebook is for teens what cable TV companies are for adults: No one likes them but everyone uses them.

Facebook doesn't break out demographic data for its monthly active users or daily active users. But broadly, all those metrics are still growing in the U.S., although they're topping out as the number of monthly active users approaches 200 million.

His Instagram comment is equally telling:

... it's just growing still quickly. I mean the number that we just said was 130 million monthly actives. Video product is growing really quickly. There are so many directions to expand this in that we think that the right focus for now is to continue just focusing on increasing the footprint of Instagram. And when the right time comes then we'll think about doing advertising.

The photosharing app has zero revenue attached to it right now because Zuckerberg is letting it grow its user base. Obviously, Instagram is much cooler than Facebook right now.

Together, that suggests Instagram is Zuckerberg's backup plan for teens: They may defect from Facebook someday, but his 1 million advertisers will still be able to reach them on Instagram as soon as he decides to flip that revenue switch to the "on" position.